Either you have a timeout value on mount or your using automounter would
be my guess. Turn debugging on the daemons and that will give more info.

From: Fong Vang

Someone is probably sitting on the mount. They don't have to actually use
it - as long as they're in the directory, it becomes unmountable. Use
utilities like fuser to determine which user is in the directory
(filesystem). There are other ways as well (the proc utilities, e.g.).

Hi,
Are these filesystems mounted via automountd ? Is this machine a NIS(+)
client ?
If yes, there is a specification for automounted nfs filesystems. The fs is
umounted after an idle time and auto-remounted if it's needed.
E.g. server:/export/foo is automounted on /export/foo.
$ mount | grep foo

Someone is probably sitting on the mount. They don't have to actually use
it - as long as they're in the directory, it becomes unmountable. Use
utilities like fuser to determine which user is in the directory
(filesystem). There are other ways as well (the proc utilities, e.g.).